1. Field of the Invention
The present invention relates to a system for producing and maintaining high purity degassed water for use in the layup or filling of nuclear power plant systems during periods of plant shutdown. The use of degassed water prevents oxidation of plant components or deposits. The degassing capability of the present invention allows operators of nuclear power plants to improve overall plant system integrity by reducing the potential for oxidation of secondary surfaces of plant components or existing deposits. This is accomplished by continuously or periodically removing oxygen and other potentially damaging gases from the layup water solution without extracting or removing the layup chemicals, such as, for example, ammonia, morpholine or DMA.
2. Description of Related Art
Electric generating stations or power plants are routinely shut down to complete routine inspection and maintenance tasks that cannot be performed during normal operation. During these periods of shutdown or outages, the plant systems are placed in various layup states, which are designed to prevent corrosion of the plant component surfaces upon exposure to air or oxygenated water. To accomplish this task, layup solutions are prepared and added to the plant systems. The use of layup solutions creates a wet layup state.
There are numerous options for wet layup solutions. For example, water at an elevated pH (typically 8.5 to 10.5 by ammonia or other amine) or water laden with an oxygen scavenger such as hydrazine or hydrazine-hydroquinone/quinhydrone (so called catalyzed hydrazine) may be used. Alternatively, deoxygenated water can be added to the plant systems. The systems are then inerted using nitrogen. Typically, the deoxygenated layup water is prepared in a special plant system employing vacuum degassing techniques, nitrogen sparging and blanketing of supply tanks, or by chemical treatment systems as disclosed, for example, in U.S. Pat. Nos. 4,818,411 and 4,556,492. In some instances, secondary system components of the power plant are placed in a “dry layup” state by passing dehumidified air through the system components. This leads to dry out and cessation of any ongoing oxidation.
In pressurized water reactor nuclear power plants, the need for prevention of the corrosion or oxidation of plant systems during layup is particularly important. The presence of oxidized species arising from improper layup states increases the risk of component corrosion during periods of subsequent operation. For example, there is a concern of steam generator tube corrosion, which could arise if the steam generator, feedwater, condensate and drain system surfaces or their overlying corrosion protection layers become oxidized during outages. These oxidized species introduce the possibility of higher electrochemical potentials in the steam generators during subsequent operation. Specifically, the formation of oxidized iron species such as hematite from the ever-present but benign magnetite deposits, and oxidized copper species such as cuprite (Cu2O) and tenorite (CuO) could promote intergranular attack or stress corrosion cracking of the nickel alloy steam generator tubes. These tubes not only serve as the surfaces through which heat from the reactor is transferred to the secondary plant, but also represent a boundary between the radioactive primary system and the non-radioactive secondary system. Breaches in this boundary due to corrosion increase the risk of exposure by the plant staff and public to unacceptable levels of radioactivity.
A number of industry guidelines have been established regarding recommended practices for steam generator wet layup (see, e.g., for example, EPRI Report TR-112967 “Source Book on Limiting Exposure to Startup Oxidants”). The recommendations contained in these guidelines typically focus on: (1) using low oxygen fill water, (2) maintaining non-oxidizing conditions, (3) maintaining strongly reducing conditions, (4) performing remedial “hot soaks” or conditioning steps during startup to reduce any oxidized species that may have formed during the outage. Each of these approaches has some limitations or disadvantages.
First, a supply of deoxygenated fill water is typically not a problem at a given power plant, but experience suggests that once a system is partially filled, the liquid will tend to gradually absorb oxygen from air whenever free surfaces are available. Second, the general approach to maintaining non-oxidizing (reducing) conditions is to raise the pH of the water and add an oxygen scavenger such as hydrazine. Unfortunately, recent tests have demonstrated that even at elevated pH, and with hydrazine present, copper in stream generator deposits can undergo conversions as high as 0.25% in five days at ambient temperature, as reported in EPRI Report TR-1001204 “Oxidation and Reduction of Copper in Steam Generator Deposits,” September 2001. Lab test data has demonstrated that significant increases in electrochemical potential, and therefore corrosion can occur with as little as 0.1% copper oxides, as reported in EPRI Report NP-6721-SD “Corrosion Evaluation of Thermally Treated Alloy 600 Tubing in Primary and Faulted Secondary Side Environments.” Consequently, even under the best conditions, wet layup of pressure water reactor steam generators can increase risk of tube corrosion and therefore boundary leakage.
The ability to maintain both low oxygen content and strong reducing conditions during layup is beneficial. One method involves sparging the steam generators with nitrogen after addition of wet layup chemicals to displace any oxygen that is absorbed. While this approach is effective, it suffers from three disadvantages. First, plant nitrogen systems are not always available due to the need to also perform maintenance on these systems during the outage. In these cases, a portable nitrogen system including a nitrogen tanker and evaporator must be brought to the site. Second, sparging with nitrogen displaces oxygen in the upper part of the steam generator. This renders the upper region of the steam generator (or open volumes in any plant system under layup) inhabitable due to risk of asphyxiation. Consequently certain secondary side maintenance activities cannot be completed in parallel with the layup. Third, the nitrogen sparging is effective at displacing oxygen in the tube bundle of the generator, but the annulus region of the generator may still be subject to absorption of oxygen.
To maintain the necessary low oxygen levels in the layup water (typically less than 200 ppb oxygen but preferably less than 50 ppb oxygen), the water can be treated on a continuous or semi-continuous basis. These treatment strategies include the use of catalyzed hydrazine, hydrazine-activated carbon beds followed by filtration and resin treatment, and vacuum degassing of the entire system. While each is a potential solution to the problem of oxygenation of the water, none has proven to be effective or practicable. For instance, the addition of catalyzed hydrazine is more effective than hydrazine alone at typical layup temperatures (ambient), but it is costly and not proven to be effective for the prevention of deposit oxidation.
The use of a system employing hydrazine-carbon-resin beds as a means of generating deoxygenated water is discussed extensively in U.S. Pat. No. 4,818,411. Incorporating such a system, however, into a recirculation system attached to a steam generator or other secondary plan system such as the condenser or feedwater heater train would result in removal of beneficial chemical additives such as ammonia, morpholine, ETA or DMA (these amines are used to increase the water pH in accordance with the goal of maintaining reducing conditions and lowering oxidation rates for both copper and magnetite). Finally, vacuum degassing can in principal be achieved, but requires complete isolation of a system, which is not designed for vacuum operation. Vacuum degassing system pumps are also quite large and unwieldy, and would be difficult to deploy inside the tight confines of a pressure water reactor containment. Also, the process of vacuum degassing can be quite slow if the depth of the vessel is large, which often occurs when a large vertical steam generator is in layup (10 meters depth or more).
An obvious benefit would therefore be realized if a system were available for maintaining the dissolved oxygen concentration in the layup water at low levels without removing beneficial additives.